The ACS one is probably bigger because it spawns stuff on its on, i.e. goes off to stuff the robot detection table, starts checking for things to spam users with, etc. In other words, it's not quiescent even if you aren't doing anything with it yourself.
Since AOLserver's threaded, it doesn't have to use inter-process shared memory to cache stuff that's visible to each thread. I'm not aware of how top or other linux utilities compute the various memory parameters for a threaded program. Threads in Linux are kernel threads, implemented as a kind of light-weight process (this is why they're visible in the process table to top etc), and I've never dug into the details.
So I can't be of much help other than pointing out that the ACS rolls up its sleeves and gets to work right away when you fire it up.
